Filling level meter for membrane gasometers

ABSTRACT

Filling level meter ( 4 ) for membrane gasometer ( 1 ), including at least one accumulation chamber (C 1 ) arranged for containing a gas, and an impermeable flexible membrane ( 2 ) capable of moving between two positions corresponding to the chamber being totally full and totally empty, wherein the meter ( 4 ) includes:
         an instrument ( 12 ) associated with a fixed element ( 3, 9 ), arranged for generating a signal of measurement of a physical value that can be correlated to the quantity of gas contained in the chamber (C 1 );   elements for processing and displaying ( 5 ) the value of the quantity of gas corresponding to the generated signal; a filiform, flexible element ( 6 ) for connecting the instrument ( 12 ) and the membrane ( 2 ); the filiform, flexible element ( 6 ) including at least one portion constituted by an elastic member ( 7 ).

The present invention relates to the membrane gasometer sector,particularly usable for the storage of biogas (biological gas) generatedby the digestion of sludge, sewage and organic substances in general, orfor the storage of gas emanating from accumulation tanks for sewagecontaining fermentable material.

More specifically, the invention relates to a level meter suitable forcontinuously measuring the quantity of gas or biogas contained in saidgasometers or gasholders.

According to known art, membrane gasometers consist of a closed shellmade of an airtight membrane.

An improved technique is used in the manufacture of pressostatic-typemembrane gasometers, which comprise a first inner membrane (gasmembrane), which delimits a gas accumulation chamber, and a second moreexternal membrane (air membrane), arranged for creating a pressurisationchamber between the two membranes, generally filled with air, adjacentto said gas accumulation chamber.

The accumulation chamber is connected to inlet and outlet pipes for thegas contained therein, or it communicates directly with a tank below forthe storage of liquids or sludges, from which said gas emanates.

The pressurisation chamber, on the other hand, is connected to anauxiliary air ventilator or to a compressor, enabling a given pressureto be maintained inside the chamber.

In a common type of pressostatic gasholders, the first membrane forms adome above a base area, and the second membrane forms an outer dome thatencloses the first one.

Said base area can be a concrete base, for example, lined with anothermembrane attached to the first membrane along the edge, or the samesurface of a liquid in which the edge of the first membrane is immersed.

Most traditional membrane gasometers generally have a hemisphericaldome, spherical cap, “three quarter sphere” or cylindrical dome shape,the latter having a base area with a substantially rectangular orelliptical shape.

Membranes are flexible, generally made of a fabric of polyester fibresspread over a plastic material, for example PVC, and must be wellanchored to the ground or to a specific fixed structure.

For the correct use of said gasometers, it is necessary to know theextent of filling of the gas accumulation chamber, both for themanagement and regulation of the gasholder, during normal operation, andfor safety purposes. Since the second membrane encloses the firstexternally, this is not possible with a simple visual inspection, andrequires the use of a specific measuring device.

According to known art, there are two different types of measuringdevices for this purpose: a first type comprises instruments that can bedefined as “distance measurement” devices, while a second type comprisesinstruments that can be defined as “force measurement” devices.

The first type of instrument enables to measure the distance between thetop of the first membrane and the corresponding top of the secondmembrane. As a matter of fact, the latter generally maintains the sameform, since it is pressurised, while the first membrane rises and fallsdepending on the quantity of gas it contains; consequently, themeasurement of the distance between the tops of the membranes gives avalue that can be easily correlated geometrically to the quantity of gascontained in the accumulation chamber.

To this aim, patent application FR 2 766 255 describes the use of anecho instrument, generally an ultrasonic probe, associated with theouter membrane, arranged for emitting wave impulses in the direction ofthe inner membrane. Said impulses strike the inner membrane and arereflected and re-directed back towards the emitting instrument: the timetaken for the waves to go and return is processed and converted into anelectrical signal proportional to the measurement of the distancebetween the two membranes, and then correlated to a value of the volumeof gas contained in the accumulation chamber, expressed as a percentageor as an absolute value, and displayed on a specific digital indicatorpanel capable of generating a voltage or current signal, generally 0-12volts or 4-20 mA. This echo instrument is often used with a flatmetallic disc, a sort of target, placed on the top of the innermembrane, and intended as a safe reflective surface for the ultrasonicwaves.

A second “distance measurement” instrument is described in patent EP 0333 698, which describes a system comprising a reel stably fixed to thetop of the outer membrane of the gasometer, from which a wire unwinds,the end thereof being fixed to the top of the inner membrane. The reelis provided with a torsional spring arranged for rewinding the wireitself. As the wire unwinds or rewinds, following the movements of theinner membrane as it rises or falls as the gas content varies, apotentiometer records the rotation of the reel and thus gives ameasurement of the distance between the tops of the membranes, which canalso be correlated to the volume of gas contained in the accumulationchamber and displayable on a suitable digital indicator panel capable ofgenerating a suitable electrical signal.

This first type of device for measuring the quantity of gas contained inthe accumulation chamber of gasometers has certain disadvantages, commonto both applications described above.

The gas membrane, since it is made of a non-stretch, non-elastic cloth,when the accumulation chamber empties, collapses and falls in on itself,forming folds and loops. In addition, the shape assumed by theaccumulation chamber during each gas emission cycle is different: thisleads to a non bijection between the values recorded by the measurementinstruments and the actual volume of gas present in the chamber.

This disadvantage is accentuated if the gasometer is not a hemisphericaldome or spherical cap type gasometer. In particular, if the gasometercomprises a ¾ sphere dome, the gas membrane, when the accumulationchamber empties, as well as collapsing and creating loops, may becomeunbalanced and fall outside its projected area, leading to even moreimprecise measurements.

Furthermore, the weight concentrated on the first membrane of the flatmetallic disc used in association with the ultrasonic probe has anegative influence on the shape of the gas accumulation chamber, thuscausing unbalance of the same membrane when the chamber empties: themembrane tends to fall in an unbalanced, uncentered manner with respectto the projected area of the dome, thus leading to an imprecise volumereading subject to casual errors that are different for each cycle andtherefore cannot be standardised. In an attempt to remedy this problem,the cited patent suggests the use of connecting cables between the firstand second membranes, arranged regularly along the maximum circumferenceof the spherical surface. Said solution, although preventing the loss ofbalance, disadvantageously distorts the volume reading: when the disctouches the surface on which the gasometer rests, the gasometer is notempty, but inside the accumulation chamber, in the remainingsemi-toroidal shape area that forms around the disc, there remains aconsiderable quantity of gas. Inside the gasometer, therefore, thevolumetric value recorded and used for the storage capacity isconsiderably lower than the actual rated geometric volume.

The main disadvantage of using a cable fixed to the top of the innermembrane is that the continuous cycles of winding and unwinding thecable tend, over time, to damage the torsional spring used to rewind thecable and, consequently, the device can no longer ensure precisereadings of the recorded value. Lastly, the weight of the gas membraneis always greater than the cable retraction force provided by theelasticity of the torsional spring in the reel: this leads to theinevitable unbalancing of the membrane when the accumulation chamberempties, with the creation of a peripheral volume of gas that cannot bedetected.

As previously mentioned, there exists a second type of instrument formeasuring the quantity of gas accumulated in gasometers, which can bedefined as “force measurement” devices, which comprise means ofmeasuring the weight of a flexible element hooked onto the outermembrane of the chamber of pressurisation air and supported by the gasmembrane. These instruments correlate a dynamometric measurement of theweight force to the quantity of gas contained.

Patent application EP 1 338 843 from the same applicant describes theuse of a measurement instrument of such a type, comprising a chain, oneend thereof hanging from a load cell type device applied to the top ofthe outer membrane, while the other end is free, and simply rests on thetop of the gas membrane. A raised edge is placed on the top of said gasmembrane, arranged for delimiting a narrow zone on which said chainrests and in which it is contained.

The load cell measures the overall weight of the chain that is notsupported by the gas membrane and provides an electrical signal. Whenthe accumulation chamber is partially or completely filled with gas, apart of the chain rests on the top of said chamber and the load cellmeasures the traction due only to the weight of the part of the chainthat remains suspended. In this way, the electrical signal generated bythe load cell provides an indirect measurement of the height of the topof the accumulation chamber.

Advantageously, this measurement can be easily correlated to thequantity of gas stored, besides the raised edge keeps the chain restingon the central area of the top of the accumulation chamber, preventingthe chain from sliding sideways, which could compromise the precision ofthe measurement.

However, for the measurement of the quantity of gas present, this secondsolution also has several disadvantages.

The chain also constitutes, although to a lesser extent than themetallic disc used in association with the ultrasonic probe, a load onthe central portion of the inner membrane, which is thus still subjectto the risk of becoming unbalanced and falling in an uncentered mannerwith respect to the projected area of the dome of the gasometer and,disadvantageously, there remains the risk of erroneous and imprecisemeasurements by the load cell due to the effect of the residual volumeremaining in the peripheral area when the central zone of the membranehas already reached the base plane, corresponding to the maximumdistance away from the top of the outer membrane.

Aim of the present invention is to eliminate all the problems anddisadvantages described above and found in all the various measurementtechniques traditionally used.

Main aim of the present invention is to provide a device for membranegasholders arranged for continuously measuring the quantity of gascontained in the accumulation chamber, providing a precise measurement,with a unique correspondence between the value measured by theinstrument and the real quantity of gas contained in the accumulationchamber.

Specifically, aim of the invention is to control the fall of the gasmembrane so that the membrane itself does not become unbalanced, andensuring that emptying of the accumulation chamber starts from theperipheral areas of the gasometer, while the top of the membrane is thelast portion of the membrane to touch the base plane.

A further aim of the present invention is to provide a measurementdevice that does not use a non-stretching mechanical connection betweenthe two membranes, thereby avoiding the risk of tears or lacerationscaused by relative movements between the membranes themselves.

Lastly, a further aim of the present invention is to provide a devicefor measuring the quantity of gas contained in the accumulator that issimple and economical to manufacture, easy to install and gives precisereadings of the values recorded.

These aims are achieved with a filling level meter for membranegasometers consisting of at least one accumulation chamber arranged forcontaining a gas, comprising an impermeable, flexible membrane capableof moving between two positions corresponding to the chamber beingtotally full and totally empty, wherein said meter comprises:

-   -   an instrument associated with a fixed element, arranged for        generating a signal of measurement of a physical value that can        be correlated to the quantity of gas contained in said chamber;    -   means for processing and displaying the value of the quantity of        gas corresponding to said generated signal;    -   a filiform, flexible means for connecting said instrument and        said membrane;

ccharacterized in that said filiform, flexible means comprises at leastone portion constituted by an elastic means.

According to a first preferred embodiment of the invention, saidinstrument is a device arranged for detecting a tractive force, and inparticular is a dynamometer or load cell.

According to further preferred embodiments of the invention, saidfiliform means comprises a helical spring, a plurality of helicalsprings arranged in series or in parallel, or in sequence with respectto a mechanical traction organ, such as a heavy chain, for example, i.e.with a weight per unit length comparable to the tractive force developedby the elastic means.

According to a further embodiment of the invention, said filiform meanscomprises two ends arranged along the vertical direction established bythe force of gravity g, and in particular arranged so that the distancebetween two ends thereof is a minimum when the accumulation chamber iscompletely filled.

The main advantage obtained with this invention is the possibility ofregulating the fall of the gas membrane when the accumulation chamberempties, so as to prevent the formation of folds and loops and preventthe membrane from becoming unbalanced, which could distort themeasurements. This is even more advantageous for ¾ sphere gasometers,preferred because they are less bulky on the ground, but tend to becomeunbalanced more easily during emptying.

Advantageously, the invention ensures that emptying of the accumulationchamber always starts from the peripheral zones of the gasometer, whilethe top of the membrane is the last part to touch the ground, due to theupward traction that the elastic means exercises on the membrane of theaccumulation chamber.

Advantageously, the shape assumed by the accumulation chamber issubstantially the same during every cycle of filling and emptying andthe values read by the meter correspond univocally to the actual volumeof the gas present in the chamber. This results in precise, extremelyreliable measurement.

A further advantage is represented by the use of elastic means forconnecting the instrument to the gas membrane, which are useful forabsorbing stresses caused by the working cycles or by outside agents onthe gas membrane, in order to avoid tears or damage to the membrane andto prevent the risk of gas leaks.

A further advantage is represented by the ease of installation of theinstrument, and to the even easier reading of the values recorded.

These and other advantages of the invention will become more apparentfrom the following detailed description of preferred embodiments of theinvention, which is illustrated by way of non-limitative example in theaccompanying drawings, in which:

FIG. 1 shows a schematic transverse cross-section view of a membranegasholder of a pressostatic type with a % sphere shape with differentfilling levels, with a filling level meter according to the presentinvention;

FIGS. 2-4 show a front view of level meters according to possiblevariants of the invention;

FIGS. 6 and 7 show a transverse cross-section view of membranegasometers of the domed pressostatic type, in which the gas accumulationchamber is closed at the bottom by a liquid surface;

FIG. 7 shows, a transverse cross-section view of a cushion-typesingle-membrane gasometer, with a filling level meter according to thepresent invention.

With reference to FIG. 1, the gasometer 1 illustrated is a pressostatictype placed on a base B and comprised of a first inner membrane 2 and asecond outer membrane 3. Said membrane 2 delimits a gas accumulationchamber C1, while said membrane 3 delimits an air pressurisation chamberC2 included, for example, between the two membranes and the base B.

Said chamber C1 is closed at the bottom by a base area, impermeable togas. Said area consists, according to known art, of a distinctimpermeable membrane, or of the same membrane 2 closing over itself, orof impermeable rigid materials and the surface of the liquid containedin a tank.

Said chamber C1 is generally connected to known means (not shown) forthe inlet and outlet of gas, while said chamber C2 is connected to knownmeans (not shown) for blowing of pressurisation air, control andpressure regulation.

Both membranes 2 and 3 are anchored to a base B by means of mechanicalfastenings (not shown) according to known art.

On the top of the gasometer 1, there is a level meter 4 according to theinvention, which enables a measurement of the quantity of gas containedin said accumulation chamber C1.

With reference to FIGS. 2, 3 and 4, said level meter comprises aninstrument 12 associated with said second membrane 3 by means of aflange 13 and counter-flange 14 connection, and connected electricallyby means of a cable 15 to remote means 5 for processing and displayingan electrical signal generated by the values recorded by said instrument12.

A filiform, flexible means 6, of a type at least in part elastic,connects said instrument 12 to said membrane 2.

Said instrument 12, protected by a cap 16 and stably fixed to the secondmembrane 3, is arranged for generating a signal of measurement of aphysical value that can be correlated to the quantity of gas containedin the accumulation chamber C1. In particular, said instrument 12 is adevice that can continuously detect a tractive force, and is generallychosen from known devices for this purpose, such as load cells ordynamometers.

Said means 5 of processing and display, connected to the instrument 12,are arranged for receiving said electrical signal of measurement andconvert it into a value that can be displayed, as an absolute value oras a percentage, corresponding to the quantity of gas contained in thechamber C1, as well as a variable current or voltage value within afinite interval.

Said filiform, flexible means 6, arranged for connecting saidmeasurement instrument 4 to said membrane 2, comprises a helical spring7 with an elastic constant K, specifically chosen to stretch in order toallow the gasometer to empty, but at the same time able to support agood part of the weight of the membrane 2 without yielding.

In particular, the two ends 7′-7″ of said spring 7 are arranged alongthe direction of the force of gravity g, so that the whole spring 7takes a vertical position, perpendicular to the base B and substantiallycentered with respect to the area defined by the planimetric projectionof accumulation chamber C1.

With reference to FIG. 2, the said filiform, flexible means 6 comprisesa plurality of helical springs 7, arranged in parallel.

In particular, said springs 7 are fixed by their ends 7′-7″ to two metalplates 10-11, which are in turn connected to the membrane 2 and to theinstrument 12 respectively, so that the tractive force exerted by themembrane and detected by the instrument 12 is evenly distributed, andenabling a correct reading of the volume of gas contained in theaccumulation chamber C1.

With reference to FIG. 3, said filiform, flexible means 6 comprises aplurality of helical springs 7, arranged in series, so that they canextend, shorten and support the membrane 2 without yielding the entireelastic means 6.

The ends 7′-7″ of the outer helical springs 7 in the series, are in turnconnected to the instrument 12 and to the membrane 2 respectively.

With reference to FIG. 4, a variant of the level meter is illustrated,wherein the filiform means 6 consists of both an elastic portion, i.e. ahelical spring 7 with a suitable elastic constant K, and a heavymechanical, non-stretching traction organ, such as a chain 8, forexample, with a weight per unit length comparable to the tractive forcedeveloped by the elastic portion, arranged in series, so that the end 8′of the chain 8 is connected to the instrument 12, while the end 7″ ofthe spring 7 is connected to the membrane 2.

The filiform, flexible means 6 connecting the measuring instrument 4 andthe membrane 2, in all its variants always comprises at least oneelastic portion 7, in order to transmit to said instrument 4 tractionstresses, proportional to the distance between the two membranes, and tosupport the gas membrane.

FIGS. 5 and 6 show the application of the invention to pressostatic typemembrane gasometers 1, wherein the base area of the gas accumulationchamber is formed by the surface of a liquid L and the chamber itself ispartially enclosed by the walls of the tank. In the case of FIG. 5, themembrane 2 rests on the liquid L, while in the case of FIG. 6 it restson a net 18 (or on a wooden roof) supported by a central pole 19.

FIG. 7 shows the application of the invention to a non-pressostaticshell type gasometer 1, i.e without a compensation chamber C2, andtherefore provided with only one membrane 2, arranged for acting as anaccumulation chamber C1 with a substantially cushion-like shape, andfiliform means 17 of anchoring the gasometer to a base B or to theground. p The level meter 4, in these variants of the invention also, isconstructed in the same manner described above, with the possibility ofthe same variants.

In particular, in the case of FIG. 7, the level meter 4 can besupported, instead of by the top of the membrane 3, which is no longerpresent, but by a fixed external element 9 anchored to the ground.

According to the present invention, during pressostatic gas accumulatoroperation, the chamber C1 fills with or empties of gas, varying itsshape and volume, while the chamber C2 is maintained at a certain levelof constant pressure. The result is that the membrane 2 moves topositions between the two extreme situations, corresponding to chamberC1 being totally full and totally empty, while the membrane 3 maintainsits position and its shape, since it is stably connected to the groundand is pressurised by air blown by suitable means.

With accumulation chamber C1 totally full, the helical spring withelastic constant K, reaches its minimum elongation Lmin corresponding tothe minimum distance between the two membranes 2 and 3. In saidcondition, the instrument 4 records a tractive force equivalent to theproduct of the constant K and the minimum elongation Lmin, and using thedisplay 5 converts said force value into an absolute value or percentagecorresponding to the actual volume present in the chamber C1, which issubstantially equivalent to the maximum capacity of the gasometer 1.

With accumulation chamber C1 totally empty, the helical spring withelastic constant K, reaches its maximum elongation Lmax corresponding tothe distance between the membrane 3 and the base area, and exerts amaximum upward tractive force on the membrane 3. In said condition, theinstrument 4 records a tractive force equivalent to the product of theconstant K and the maximum elongation Lmax, and allows the conversion ofsaid force value into an absolute value or percentage corresponding tothe actual volume present in the chamber C1, which is substantiallyequivalent to zero.

During the various measurement phases, the instrument 4 will recordintermediate tractive force values between the two extreme situationsdescribed above, the means 5 will process said values, generating visualindications of volume corresponding to the actual quantities of gaspresent in the accumulation chamber C1 instant by instant, and anelectrical signal that can be correlated to the same quantity.

In the context of the same inventive idea, the helical spring 7 canclearly be replaced by an elastic element arranged for serving the samefunction, such as an elastic cable for example, while the mechanicaltraction organ 8 of the heavy chain type can be replaced bynon-stretching wires or cables, which may be weighed down by loadsarranged along their length at regular intervals.

The invention as described and illustrated achieves the set tasks andaims. In practice, it is clear that all the details regarding geometry,shape, dimension and materials details can be replaced by otherfunctionally equivalent, without straying from the scope of protectionof the claims.

1) Filling level meter (4) for membrane gasometers (1), consisting of atleast one accumulation chamber (C1) arranged for containing a gas,comprising an impermeable flexible membrane (2) capable of movingbetween two positions corresponding to the chamber being totally fulland totally empty, wherein said meter (4) comprises: an instrument (12)associated with a fixed element (3, 9), arranged for generating a signalof measurement of a physical value that can be correlated to thequantity of gas contained in said chamber (C1); means for processing anddisplaying (5) the value of the quantity of gas corresponding to saidgenerated signal; a filiform, flexible means (6) for connecting saidinstrument (12) and said membrane (2); characterized in that saidfiliform, flexible means (6) comprises at least one portion constitutedby an elastic means (7). 2) Level meter (4) according to claim 1,characterized in that said instrument (12) is a device arranged fordetecting a tractive force. 3) Level meter (4) according to claim 1,characterized in that said instrument (12) is a load cell. 4) Levelmeter (4) according to claim 1, characterized in that said instrument(12) is a dynamometer. 5) Level meter (4) according to claim 1,characterized in that said filiform means (6) comprises a helical spring(7). 6) Level meter (4) according to claim 1, characterized in that saidfiliform means (6) comprises a plurality of helical springs (7) arrangedin series or in parallel. 7) Level meter (4) according to claim 1,characterized in that said filiform, flexible means (6) comprises aheavy mechanical traction organ. 8) Level meter (4) according to claim7, characterized in that said heavy mechanical traction organ is a chain(8). 9) Level meter (4) according to claim 1, characterized in that saidfiliform, flexible means (6) has ends (7′, 7″, 8′) arranged along thedirection of the force of gravity g. 10) Membrane gasometer (1)comprising a level meter (4) according to claim
 1. 11) Level meter (4)according to claim 2, characterized in that said instrument (12) is aload cell. 12) Level meter (4) according to claim 2, characterized inthat said instrument (12) is a dynamometer.